1. Field of the Invention
The present invention relates to a radiation image processing apparatus and a processing method for producing a piece of radiation image information of a desired image capturing site by processing a plurality of pieces of radiation image information acquired by applying radiation of different energies to a subject, respectively.
2. Description of the Related Art
In the medical field, for example, extensive use is made of radiation image processing apparatuses which expose a subject (patient) to radiation emitted from a radiation source, guide the radiation that has passed through the subject to a radiation converting panel for converting radiation into radiation image information, and then perform predetermined image processing on the radiation image information. Processed radiation image information is displayed on a display unit and can be used for diagnosis etc.
One example of the radiation converting panel is a solid-state detector that converts radiation into charge information and stores the charge information so that it can be read out as an electric signal. Another example of the radiation converting panel is a stimulable phosphor panel. The stimulable phosphor panel stores radiation energy in a phosphor and emits stimulated light of an intensity corresponding to the stored energy when irradiated with stimulating light such as a laser beam.
One practical application of the radiation image apparatus is the extraction of a region of interest in the subject, e.g., soft tissue such as the heart and lungs located under ribs, from the radiation image information. The extraction of the region of interest is achieved based on a difference in absorption characteristics between bone such as the ribs, and soft tissue such as the heart. Radiation of different energies is applied to the subject using two different image capturing conditions to acquire two pieces of radiation image information. The extraction of the bone or soft tissue of interest is achieved by computing the difference between the two pieces of radiation image information after weighting with one or more predetermined coefficients (see Japanese Laid-Open Patent Publication No. 2002-325756).
Different internal structures of the subject have different radiation absorption characteristics. Therefore, it is necessary to process the image in accordance with these characteristics to acquire a proper image of the region of interest. For example, if the subject is a fracture that is applied with a fixture material such as a plaster cast, the image processing should take into consideration the radiation absorption characteristics of the plaster and bones.
The image processing method disclosed in Japanese Laid-Open Patent Publication No. 2002-330954, acquires a first piece of radiation image information by applying radiation to a subject in accordance with a predetermined image capturing condition. Then, the image capturing condition is modified based on analysis of the first piece of radiation image information, and used for acquisition of a second piece of radiation image information. Finally, an image of a region of interest is produced from the first and second pieces of radiation image information.
However, the method disclosed in Japanese Laid-Open Patent Publication No. 2002-330954, which determines the image capturing condition for the second piece of radiation image information based on the first piece of radiation image information, may fail to obtain proper image information when the subject moves before the acquisition of the second piece of radiation image information.
The fixture material applied to the subject can be plaster of a plaster cast or fiberglass, which have different radiation absorption characteristics. If plaster is used, the radiation absorption characteristic is also affected by the water content of the plaster, which decreases with solidification. The radiation absorption of the plaster cast with some water content at the beginning of application is different from that of the plaster cast which is solidified after a certain time. Therefore, it is necessary to process the pieces of radiation image information with consideration for the radiation absorption characteristics of the subject. Japanese Laid-Open Patent Publication No. 2002-330954, however, does not take into account the radiation absorption characteristics of the subject. Therefore, it may fail to acquire a proper image capturing condition from the first piece of radiation image information and determine suitable weighting coefficients for the weighted subtraction using the first and second pieces of radiation image information.